SIB byte
An optional x86-64 byte that describes a memory operand with an index register and a scale, such as [rdi + rsi*4].
The SIB (scale-index-base) byte follows ModRM when r/m = 100 with a memory mod; it has scale (2 bits, factor 1/2/4/8), index (3 bits) and base (3 bits), each register field extended by REX.X or REX.B.